This invention relates to a plural motor changeover control system which, by means of the same control apparatus, controls changeover among a plurality of motors the outputs of which differ.
In a machine tool or the like equipped with a plurality of motors having different outputs, the motors are driven alternatively. By way of example, in a turning center or the like, a turning machining motor is driven when turning machining is performed, thereby to rotate a workpiece at a commanded rotational velocity to carry out the turning operation. When a screw hole, hole or keyway is to be formed after the turning operation, a spindle is positioned at a predetermined point, upon which a motor for rotary tool is driven to rotate the tool, thereby drilling a hole into the stationary workpiece. Since the motor for the turning operation and the motor for the rotary tool are so arranged that while one is being driven the other is not, it has been contemplated to control the motors by a single servo control apparatus. If it were possible to apply such a servo control apparatus commonly for the two motors, the results would be very beneficial as it would make it possible to greatly simplify the arrangement and reduce the cost of the equipment. If the maximum outputs of the turning motor and rotary tool motor were identical, common application of the servo control apparatus would be very simple to achieve. In general, however, the maximum outputs of these motors differ by a wide margin, so that common application is difficult. Even if a servo control apparatus were to be applied to both motors, neither a significant simplification nor a significant reduction in cost would be achieved.
Moreover, in order to achieve such common application, it would be necessary to provide a switching circuit for the signal lines that transmit feedback signals from the motors, in addition to a switching circuit for the power lines that transmit the motor drive signals. These switching circuits use separately installed switching units that rely upon relay circuits. When relays are used, however, it is necessary to provide a power supply for driving the relays. This requires a great amount of space and complicated wiring.